The present invention relates generally to frequency measuring systems, and more particularly, to a method and apparatus for providing a frequency measuring system to give the best instantaneous value of the frequency, particularly when the frequency being measured is decreasing in value.
There are a number of frequency measuring systems available in today's art. In one form, the cycles of the signal being measured are counted for some unit of time to give a direct value of the frequency. At low frequencies, this method is undesirable because a long count period is needed to give adequate resolution. A somewhat more sophisticated approach to the measurement problem involves measuring the period of the waveform and inverting the value of the period to get frequency. Although this method provides better results than the direct counting method described above, it does not give the optimum results particularly when the frequency is low and is rapidly getting lower.
In the known period calculation method, the period is measured and this measured value is held until another complete period has occurred. The held value will not be changed until another time period has elapsed. If the signal is increasing in frequency, the next measured period will be shorter than the previous period and the frequency will be updated as soon as the new period is known. Thus, so long as the new period is shorter than the prior period, the frequency is being updated rapidly and no difficulties are encountered. If, however, the frequency is decreasing in value, the new period will be longer than the previous period reflecting the decrease in frequency. As soon as the period currently being measured exceeds the previous period, it is known that the frequency is lower than the value which is currently being used. This knowledge, however, does not appear in the system output until a new period is complete and, thus, it is seen that there is a time delay which represents an inaccuracy in the system and which is a deficiency in this method. In the extreme case, when the input frequency is zero, the held value will be that of the last complete period. Since the final period never terminates, the output will remain that of the last complete period and the zero frequency condition will not be indicated.